Hi Jamie,
If you are using Waves plug-ins contact Waves about an update to version 4.0 that should resolve this.
There is a known noise issue with the 4.0 release of Waves.
If you are not using Waves plugs, check the following:
First make sure you are running the latest firmware version on the mbox:
You can download the OS X Mbox firmware updater here:
http://download.digidesign.com/suppo...irmware20x.hqx
-Make sure MBox is connected directly to the computer and not through a USB Hub.
-Since most third party USB cables do not include a bulk filter (the cylindrical lump near one end of the cable) to help reduce interference, make sure that you are using the USB cable that ships with MBox.
Also, try trashing Pro Tools Preferences: Go to Users > “your user name” > Library > Preferences and trash DAE Prefs, DigiSetup.OSX, Pro Tools LE 6.0 prefs, "com.digidesign.protools.plist" and "com.apple.audio.AudioMIDISetup.plist". Then restart the CPU.
Also, make sure you have initialized your audio drive as an HFS+ volume with Disk Utility.
Hope this helps.
Mike